sensors Article
Fire Source Localization Based on Distributed Temperature Sensing by a Dual-Line Optical Fiber System Miao Sun 1,2 , Yuquan Tang 1 , Shuang Yang 1,2 , Jun Li 1 , Markus W. Sigrist 3 and Fengzhong Dong 1,2, * 1
2 3
*
Anhui Provincial Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China; [email protected] (M.S.); [email protected] (Y.T.); [email protected] (S.Y.); [email protected] (J.L.) Hefei Institutes of Physical Science, University of Science and Technology of China, Hefei 230029, China ETH Zürich, Institute for Quantum Electronics, Otto-Stern-Weg 1, CH-8093 Zurich, Switzerland; [email protected] Correspondence: [email protected]; Tel.: +86-551-6559-5001
Academic Editor: Ingolf Willms Received: 28 March 2016; Accepted: 31 May 2016; Published: 6 June 2016
Abstract: We propose a method for localizing a fire source using an optical fiber distributed temperature sensor system. A section of two parallel optical fibers employed as the sensing element is installed near the ceiling of a closed room in which the fire source is located. By measuring the temperature of hot air flows, the problem of three-dimensional fire source localization is transformed to two dimensions. The method of the source location is verified with experiments using burning alcohol as fire source, and it is demonstrated that the method represents a robust and reliable technique for localizing a fire source also for long sensing ranges. Keywords: dual-line optical fiber; fire source localization; optical fiber distributed temperature sensor
1. Introduction Fire detection techniques play an important role in various areas by providing assistance to reduce the economic and ecological damage as well as casualties [1,2]. To avoid the excessive fire damage, sensor networks composed with cameras, flammable gas detectors, and temperature gauges have been implemented [2–6]. Localizing the source is the main concern. Fire intensity and size during the fire also enable forecasting [7–10]. Richards et al. used a zone model and exhaustive search with a black and white video camera to detect, locate and size accidental fires [7,8]. Xia et al. applied a model of a dual-line gas sensor array to collect information on concentrations of gases emitted by the fire and to calculate the time delays of the signals received from various array elements [3]. However, the localization results are easily affected by any obstructions present in the area of the fire place. Nowadays, various methods based on temperature measurements have been proposed to locate fires as the temperature change is perhaps the most obvious sign when a fire occurs. The main research direction is that sensor arrays with four detectors are used to locate the fire source based on the time-delay estimation algorithm [7]. One or two sensor arrays are adopted to calculate the distance between the fire source and the sensor array with far field algorithms [5,6], and near field algorithms [11]. The proposed methods yield good results as long as the fire source is close to the sensor array. However, the reliability of the localization method may suffer in the case of a remote fire. Moreover, a large number of the sensor arrays are needed when the fire has to be monitored in a huge space. Thus it is difficult to overcome the limitations such as high construction costs, difficulty in
Sensors 2016, 16, 829; doi:10.3390/s16060829
www.mdpi.com/journal/sensors
Sensors 2016, 16, 829
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installing signal-circuits, and difficulty in reducing measurement time when taking into consideration the temperature measurement with discrete sensors. An optical fiber distributed temperature sensor (DTS) system is a kind of real-time and continuous monitoring space temperature field sensing technology [12,13]. The temperature profile can be derived based on the Raman backscattered light and the optical time-domain reflectometry (OTDR). The earliest example of distributed sensing with OTDR we could find is attributable to Dakin et al. [14]. A conventional optical fiber was used as the sensing element and the distributed sensor achieved a good spatial resolution of 3 m with sensor lengths up to 1 km. Numerous studies have been performed with the DTS, and a sensing distance >10 km has been achieved based on Raman backscattering [15]. The DTS has been widely used in many areas due to its unique advantages, such as immunity to electromagnetic fields, easiness of installing wire, remote distributed measurement compared with the conventional temperature sensors [16–20]. With an optical fiber as the sensing element, the DTS can realize temperature monitoring with long-distance and wide range, which solves the deficiency of the sensor arrays. In this study, a novel method of fire source localization based on the DTS system is presented. Two parallel sections of multimode fiber are adopted as the sensing fiber. Hot gases are produced when the fire occurs in a closed room. The hot gases rise and propagate in a circular shape along the ceiling of the room. By analyzing the temperature changes measured with the DTS system, the localization of the fire source is examined theoretically. The experimental results show that the proposed method provides a reliable fire source location for long sensing ranges. 2. Theoretical Analysis 2.1. Theory of the DTS System In a DTS system two Raman scattered components (Stokes (S) Raman scattered light and anti-Stokes (AS) Raman scattered light) are generated when a short laser pulse is launched into the optical fiber. Contrary to the Stokes Raman scattered light, the intensity of the anti-Stokes Raman scattered light strongly depends on the local fiber temperature. For the temperature determination, the anti-Stokes Raman scattered light is regarded as the signal light, while the Stokes Raman scattered light is seen as the reference light. Thus the temperature (T) dependence of the intensity ratio F(T) can be expressed as follows [21]: FpTq “
ϕ AS pTq K ν4 “ AS AS expr´ph∆ν{k B Tqsexpr´pα AS ´ αS qls ϕS pTq KS νS4
(1)
where ϕ AS pTq and ϕS pTq are the intensity of AS light and S light at temperature T, respectively, KAS and KS are coefficients concerning AS and S scattering cross-section, respectively, νAS and νS are the frequencies of AS light and S light, h is the Planck constant, ∆ν is the Raman shift in the transmission medium, and kB is the Boltzmann constant, αAS and αS are the attenuation coefficients for the incident AS and S light, respectively, l is the location along the fiber. To calculate the absolute temperature, the reference fiber coil with the fiber length l0 is maintained at a known temperature T0 . Then the final temperature T can be obtained as: 1 1 k FpTq “ ´ lnp q T T0 h∆ν FpT0 q
(2)
2.2. Theory of the Fire Source Localization A near field algorithm using the DTS for the fire source localization is proposed in this paper. As shown in Figure 1, a fire breakout in a closed room will produce lots of hot gases. The hot gases rise to the ceiling of the closed room and then propagate in circular shapes across the plane of the ceiling. The flow of the hot gases will not be perfectly circular due to the impact of weak turbulences
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and the influence of the room walls. However, an almost circular gas flow is expected if a time average is taken. To simplify the problem, some assumptions are made. The ceiling of the closed room should Sensors 2016, 16,with 829 a low thermal conductivity. All walls are considered to have the same temperature. 3 of 12 be smooth Sensors 2016, 16, 829 3 of 12 The fire is located below the ceiling and not directly at a wall. The flow velocity of the hot gases is constant under the ceiling in theceiling early stage ofearly the fire. Other currents such as air currents caused by an considered constant under in theof stage of thecurrents fire. Other such as caused air currents constant under the ceiling inthe the early stage the fire. Other suchcurrents as air currents by an air conditioning system are neglected [5,11]. Under these assumptions the wavefront of the hot gases caused by an airsystem conditioning system are neglected Under these assumptions the of air conditioning are neglected [5,11]. Under [5,11]. these assumptions the wavefront ofwavefront the hot gases has a perfect circular shape. The fire source is situated at the zone underneath the center point of the the hot gases has a perfect circular shape. The fire source is situated at the zone underneath the center has a perfect circular shape. The fire source is situated at the zone underneath the center point of the circular shape and the coordinates of the center point correspond to the fire’s coordinates in the two point ofshape the circular shape and the coordinates the center point correspond the fire’s coordinates circular and the coordinates of the centerofpoint correspond to the fire’stocoordinates in the two dimensional plane. As a consequence, the problem of the three-dimensional fire source localization is in the two dimensional plane. As a consequence, problem of the three-dimensional fire sourceis dimensional plane. As a consequence, the problem the of the three-dimensional fire source localization essentially converted into a two-dimensional orientation determination of the center point of the flow localization is essentially into a two-dimensional orientationofdetermination of the center essentially converted into aconverted two-dimensional orientation determination the center point of the flow ofpoint the hot gases. Obviously a more complex model would be needed if, e.g., the fire would be located of the flow of the hot gases. Obviously a more complex model would be needed if, e.g., the fire of the hot gases. Obviously a more complex model would be needed if, e.g., the fire would be located near a wall or for non-symmetric configurations. However, as is demonstrated here, the proposed would be located a wall or forconfigurations. non-symmetric However, configurations. as is demonstrated here, near a wall or for near non-symmetric as is However, demonstrated here, the proposed method enables a fast and efficient fire source location in many situations. the proposed method enables a fast and efficient fire source location in many situations. method enables a fast and efficient fire source location in many situations. Ceiling Ceiling
Fire source Fire source
Figure 1. 1. A fire in aa closed closed room. Figure A fire fire in in Figure 1. A a closed room. room.
The two-dimensional fire source localization and geometrical arrangement of the optical fiber are The geometrical arrangement arrangementof ofthe theoptical opticalfiber fiberare are Thetwo-dimensional two-dimensionalfire firesource source localization localization and geometrical shown in Figure 2, which is the top view of Figure 1, for two different configurations. The closed rings shown for two two different differentconfigurations. configurations.The Theclosed closedrings rings shownininFigure Figure2,2,which whichisisthe the top top view view of Figure 1, for represent wavefronts of the hot gases, and each ring means a wavefront with identical temperature. represent means aa wavefront wavefrontwith withidentical identicaltemperature. temperature. representwavefronts wavefrontsofofthe thehot hot gases, gases, and and each ring means Point A is the fire source location. The wavefronts M and N represent two different radii of the circular Point represent two twodifferent differentradii radiiofofthe thecircular circular PointAAisisthe thefire firesource sourcelocation. location. The The wavefronts wavefronts M and N represent wavefront. Two sections of the optical fibers are placed parallel within the temperature field, crossing wavefront. Two sections of the optical fibers are placed parallel within the temperature field, crossing wavefront. Two sections of the optical parallel within the temperature field, crossing with different radii of wavefronts. The points B and D are the points of tangency between the fibers and with The points B and DD areare thethe points of of tangency between thethe fibers and withdifferent differentradii radiiofofwavefronts. wavefronts. The points B and points tangency between fibers the corresponding wavefronts. The assumption is made that the fire source is small enough so that it the corresponding wavefronts. The assumption is made that the fire is small enough so that and the corresponding wavefronts. The assumption is made that thesource fire source is small enough soit can be seen as a point fire source. Then the points with maximum temperature measured along the can asseen a point source. Then Then the points withwith maximum temperature measured along the thatbeit seen can be as a fire point fire source. the points maximum temperature measured along fibers are points B and D, respectively, and the smaller the radius, the higher the temperature. The fibers are points B and and and the smaller the the radius, the the higher the the temperature. The the fibers are points B D, andrespectively, D, respectively, the smaller radius, higher temperature. position of the high temperature point can be determined by using OTDR, which is the abscissa of the position of theofhigh temperature pointpoint can be by using OTDR, which is the abscissa of of the The position the high temperature candetermined be determined by using OTDR, which is the abscissa fire source. fire thesource. fire source. a a
y y
D D
N N
x x
M M
Fiber Fiber
Ⅱ
d d
Source A Source A
s= s=dd-r -r
r r B B
b b
Ⅱ
l l
y y C C
Fiber Ⅰ Fiber Ⅰ
N N
x x
M M
Source A Source A
r r
s= s=rr+d +d
B dB
l l
d
D D
C C
Fiber Ⅰ Fiber Ⅰ Fiber Fiber
Ⅱ Ⅱ
Figure 2. Fire source and the geometrical arrangement of the sensing fibers: (a) Fibers on opposite sides Figure 2.2.Fire source and thethe geometrical arrangement of the sensing fibers: (a) Fibers on opposite sides Fire and ofFigure fire source; (b)source Both fibers ongeometrical same side ofarrangement fire source. of the sensing fibers: (a) Fibers on opposite ofsides fire source; (b) Both fibers on same side of fire source. of fire source; (b) Both fibers on same side of fire source.
To determine the fire source location the ordinate of the fire source is required, which is the To determine the fire source location the ordinate of the fire source is required, which is the distance r between fiber I and the fire source. The measured temperature at point B is higher than the distance r between fiber I and the fire source. The measured temperature at point B is higher than the one at point D due to the smaller radius r of wavefront M compared to S of wavefront N. Obviously, one at point D due to the smaller radius r of wavefront M compared to S of wavefront N. Obviously, there is one point C at fiber I at which the measured temperature is the same as at point D at fiber II, there is one point C at fiber I at which the measured temperature is the same as at point D at fiber II, because points C and D belong to the same wavefront N. The distance between the two parallel fibers is because points C and D belong to the same wavefront N. The distance between the two parallel fibers is
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To determine the fire source location the ordinate of the fire source is required, which is the distance r between fiber I and the fire source. The measured temperature at point B is higher than the one at point D due to the smaller radius r of wavefront M compared to S of wavefront N. Obviously, there is one point C at fiber I at which the measured temperature is the same as at point D at fiber II, because points C and D belong to the same wavefront N. The distance between the two parallel fibers is d and l Sensors 829 is the2016, fiber16,length between point B and point C. Noticing the right triangle ABC the relationship can4 of be12 expressed as: d and l is the fiber length between point B and 2point2 C. Noticing the right triangle ABC the relationship r ` l “ s2 (3) can be expressed as: When the fire source is located between the two fibers (see Figure 2a), then: (3) r 2 l 2 s2 d´ r “fibers s When the fire source is located between the two (see Figure 2a), then:
d of rthe stwo fibers (see Figure 2b) we have: When the fire source is located on one side When the fire source is located on one side of the two fibers (see Figure 2b) we have: r`d “ s rd s Thus the distance radius r can be expressed as: Thus the distance radius r can be expressed as: ˇ 22 ˇ ˇl ´ dd2 2ˇˇ ˇ rr “ ˇ ˇ 2d 2d
(4)
(4) (5) (5)
(6) (6)
Withthe theabove aboveabscissa abscissaand andthe theordinate ordinateofofthe thefire firesource, source,the thelocalization localizationofofthe thefire firesource sourceis With is defined. defined. Experimental Setup 3.3.Experimental Setup Theschematic schematicdrawing drawing DTS system is presented in Figure 3. short The short of from light a The of of thethe DTS system is presented in Figure 3. The pulsepulse of light from afiber pulsed laser operating 1550a nm with a peak maximum W, width 10 ns pulse pulsed laserfiber operating at 1550 nmatwith maximum powerpeak of 30power W, 10 of ns 30 pulse and 10 width and 10 kHz repetition rate is injected into the optical sensing fiber through a 1 ˆ 3 wavelength kHz repetition rate is injected into the optical sensing fiber through a 1 × 3 wavelength division division multiplexer (WDM).scattering Raman scattering in thesensing optical sensing fiber, the backscattered multiplexer (WDM). Raman occurs inoccurs the optical fiber, and theand backscattered Raman RamanasStokes the Raman anti-Stokes passing through WDM injectedinto into a Stokes well as as well the as Raman anti-Stokes light light passing through the the WDM areareinjected a high-performance InGaAs avalanche photodiode (APD). Using the APD, the Raman Stokes high-performance InGaAs avalanche photodiode (APD). Using the APD, the Raman Stokesand and anti-Stokeslight lightare aretransformed transformed into into electrical electrical signals. signals. Then anti-Stokes Then aa high high speed speed100 100MHz MHzdata dataacquisition acquisition cardconverts convertsthe theanalog analogsignals signals to to digital digital signals. card signals. Finally, Finally, the thecomputer computerprocesses processesthe thedigital digitalsignals signals and calculates the temperature data according to the theoretical analysis presented in Section 2.1. and calculates the temperature data according to the theoretical analysis presented in Section 2.1. Laser
1×3 WDM Sensing Fiber
APD
SYNC
APD
DAQ
Figure setup (WDM: (WDM: wavelength wavelengthdivision divisionmultiplexer; multiplexer;APD: APD:InGaAs InGaAs Figure3.3.Schematic Schematicof ofthe the DTS DTS system system setup avalanche photodiode; DAQ: data acquisition). avalanche photodiode; DAQ: data acquisition).
To verify the theoretical analysis fire source localization experiments were carried out in a To verify the theoretical analysis fire source localization experiments were carried out in a laboratory in a simulated environment. The experimental setup is shown in Figure 4. A tray of alcohol laboratory in a simulated environment. The experimental setup is shown in Figure 4. A tray of (11.0 cm × 8.0 cm × 2.1 cm) placed on a table (2.4 m × 1.2 m × 0.8 m) is used as the fire source. In the alcohol (11.0 cm ˆ 8.0 cm ˆ 2.1 cm) placed on a table (2.4 m ˆ 1.2 m ˆ 0.8 m) is used as the fire closed laboratory room, the hot gases rise to the ceiling and propagate under the ceiling. Then the hot source. In the closed laboratory room, the hot gases rise to the ceiling and propagate under the gases flow down at the walls and flow back to the fire source origin near floor level. Thus there is a hot flue gas layer with a thickness of 100–300 mm beneath the ceiling if the fire source is located far away from the walls. In order to simplify the experiments, a heat insulation roof (2.4 m × 1.2 m × 0.05 m) supported by four pillars is placed 1 m above the table. This roof is placed parallel to the table and acts as the “ceiling” as discussed above in Figure 1. The table, the four pillars and the ceiling form an open
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ceiling. Then the hot gases flow down at the walls and flow back to the fire source origin near floor level. Thus there is a hot flue gas layer with a thickness of 100–300 mm beneath the ceiling if the fire source is located far away from the walls. In order to simplify the experiments, a heat insulation roof (2.4 m ˆ 1.2 m ˆ 0.05 m) supported by four pillars is placed 1 m above the table. This roof is placed parallel to the table and acts as the “ceiling” as discussed above in Figure 1. The table, the four pillars and the ceiling form an open cabinet in the laboratory. If a fire occurs in the cabinet, the hot gases produced by the fire rise up to the heat insulation roof and propagate under the roof, forming a layer of the2016, hot16, flue of12 Sensors 829gases beneath the roof. Because the open cabinet is placed far away from the walls 5 of the closed laboratory room, the hot gases propagating to the edge of the roof will continue to flow horizontally of flowing thethe room walls. There is layer no influence on roof. the when arrivinginstead at the room walls.down Therewhen is no arriving influenceaton inner hot flue gas under the inner hot flue layer is under the roof. As a result, the cabinet is of employed to simulate true situation As a result, thegas cabinet employed to simulate a true situation a fire with sensors aplaced far away of a the fire edge with sensors placed far awaymultimode from the edge the roof. A standard with from of the roof. A standard fiberofwith an attenuation ofmultimode
Fire Source Localization Based on Distributed Temperature Sensing by a Dual-Line Optical Fiber System Miao Sun 1,2 , Yuquan Tang 1 , Shuang Yang 1,2 , Jun Li 1 , Markus W. Sigrist 3 and Fengzhong Dong 1,2, * 1
2 3
*
Anhui Provincial Key Laboratory of Photonic Devices and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China; [email protected] (M.S.); [email protected] (Y.T.); [email protected] (S.Y.); [email protected] (J.L.) Hefei Institutes of Physical Science, University of Science and Technology of China, Hefei 230029, China ETH Zürich, Institute for Quantum Electronics, Otto-Stern-Weg 1, CH-8093 Zurich, Switzerland; [email protected] Correspondence: [email protected]; Tel.: +86-551-6559-5001
Academic Editor: Ingolf Willms Received: 28 March 2016; Accepted: 31 May 2016; Published: 6 June 2016
Abstract: We propose a method for localizing a fire source using an optical fiber distributed temperature sensor system. A section of two parallel optical fibers employed as the sensing element is installed near the ceiling of a closed room in which the fire source is located. By measuring the temperature of hot air flows, the problem of three-dimensional fire source localization is transformed to two dimensions. The method of the source location is verified with experiments using burning alcohol as fire source, and it is demonstrated that the method represents a robust and reliable technique for localizing a fire source also for long sensing ranges. Keywords: dual-line optical fiber; fire source localization; optical fiber distributed temperature sensor
1. Introduction Fire detection techniques play an important role in various areas by providing assistance to reduce the economic and ecological damage as well as casualties [1,2]. To avoid the excessive fire damage, sensor networks composed with cameras, flammable gas detectors, and temperature gauges have been implemented [2–6]. Localizing the source is the main concern. Fire intensity and size during the fire also enable forecasting [7–10]. Richards et al. used a zone model and exhaustive search with a black and white video camera to detect, locate and size accidental fires [7,8]. Xia et al. applied a model of a dual-line gas sensor array to collect information on concentrations of gases emitted by the fire and to calculate the time delays of the signals received from various array elements [3]. However, the localization results are easily affected by any obstructions present in the area of the fire place. Nowadays, various methods based on temperature measurements have been proposed to locate fires as the temperature change is perhaps the most obvious sign when a fire occurs. The main research direction is that sensor arrays with four detectors are used to locate the fire source based on the time-delay estimation algorithm [7]. One or two sensor arrays are adopted to calculate the distance between the fire source and the sensor array with far field algorithms [5,6], and near field algorithms [11]. The proposed methods yield good results as long as the fire source is close to the sensor array. However, the reliability of the localization method may suffer in the case of a remote fire. Moreover, a large number of the sensor arrays are needed when the fire has to be monitored in a huge space. Thus it is difficult to overcome the limitations such as high construction costs, difficulty in
Sensors 2016, 16, 829; doi:10.3390/s16060829
www.mdpi.com/journal/sensors
Sensors 2016, 16, 829
2 of 12
installing signal-circuits, and difficulty in reducing measurement time when taking into consideration the temperature measurement with discrete sensors. An optical fiber distributed temperature sensor (DTS) system is a kind of real-time and continuous monitoring space temperature field sensing technology [12,13]. The temperature profile can be derived based on the Raman backscattered light and the optical time-domain reflectometry (OTDR). The earliest example of distributed sensing with OTDR we could find is attributable to Dakin et al. [14]. A conventional optical fiber was used as the sensing element and the distributed sensor achieved a good spatial resolution of 3 m with sensor lengths up to 1 km. Numerous studies have been performed with the DTS, and a sensing distance >10 km has been achieved based on Raman backscattering [15]. The DTS has been widely used in many areas due to its unique advantages, such as immunity to electromagnetic fields, easiness of installing wire, remote distributed measurement compared with the conventional temperature sensors [16–20]. With an optical fiber as the sensing element, the DTS can realize temperature monitoring with long-distance and wide range, which solves the deficiency of the sensor arrays. In this study, a novel method of fire source localization based on the DTS system is presented. Two parallel sections of multimode fiber are adopted as the sensing fiber. Hot gases are produced when the fire occurs in a closed room. The hot gases rise and propagate in a circular shape along the ceiling of the room. By analyzing the temperature changes measured with the DTS system, the localization of the fire source is examined theoretically. The experimental results show that the proposed method provides a reliable fire source location for long sensing ranges. 2. Theoretical Analysis 2.1. Theory of the DTS System In a DTS system two Raman scattered components (Stokes (S) Raman scattered light and anti-Stokes (AS) Raman scattered light) are generated when a short laser pulse is launched into the optical fiber. Contrary to the Stokes Raman scattered light, the intensity of the anti-Stokes Raman scattered light strongly depends on the local fiber temperature. For the temperature determination, the anti-Stokes Raman scattered light is regarded as the signal light, while the Stokes Raman scattered light is seen as the reference light. Thus the temperature (T) dependence of the intensity ratio F(T) can be expressed as follows [21]: FpTq “
ϕ AS pTq K ν4 “ AS AS expr´ph∆ν{k B Tqsexpr´pα AS ´ αS qls ϕS pTq KS νS4
(1)
where ϕ AS pTq and ϕS pTq are the intensity of AS light and S light at temperature T, respectively, KAS and KS are coefficients concerning AS and S scattering cross-section, respectively, νAS and νS are the frequencies of AS light and S light, h is the Planck constant, ∆ν is the Raman shift in the transmission medium, and kB is the Boltzmann constant, αAS and αS are the attenuation coefficients for the incident AS and S light, respectively, l is the location along the fiber. To calculate the absolute temperature, the reference fiber coil with the fiber length l0 is maintained at a known temperature T0 . Then the final temperature T can be obtained as: 1 1 k FpTq “ ´ lnp q T T0 h∆ν FpT0 q
(2)
2.2. Theory of the Fire Source Localization A near field algorithm using the DTS for the fire source localization is proposed in this paper. As shown in Figure 1, a fire breakout in a closed room will produce lots of hot gases. The hot gases rise to the ceiling of the closed room and then propagate in circular shapes across the plane of the ceiling. The flow of the hot gases will not be perfectly circular due to the impact of weak turbulences
Sensors 2016, 16, 829
3 of 12
and the influence of the room walls. However, an almost circular gas flow is expected if a time average is taken. To simplify the problem, some assumptions are made. The ceiling of the closed room should Sensors 2016, 16,with 829 a low thermal conductivity. All walls are considered to have the same temperature. 3 of 12 be smooth Sensors 2016, 16, 829 3 of 12 The fire is located below the ceiling and not directly at a wall. The flow velocity of the hot gases is constant under the ceiling in theceiling early stage ofearly the fire. Other currents such as air currents caused by an considered constant under in theof stage of thecurrents fire. Other such as caused air currents constant under the ceiling inthe the early stage the fire. Other suchcurrents as air currents by an air conditioning system are neglected [5,11]. Under these assumptions the wavefront of the hot gases caused by an airsystem conditioning system are neglected Under these assumptions the of air conditioning are neglected [5,11]. Under [5,11]. these assumptions the wavefront ofwavefront the hot gases has a perfect circular shape. The fire source is situated at the zone underneath the center point of the the hot gases has a perfect circular shape. The fire source is situated at the zone underneath the center has a perfect circular shape. The fire source is situated at the zone underneath the center point of the circular shape and the coordinates of the center point correspond to the fire’s coordinates in the two point ofshape the circular shape and the coordinates the center point correspond the fire’s coordinates circular and the coordinates of the centerofpoint correspond to the fire’stocoordinates in the two dimensional plane. As a consequence, the problem of the three-dimensional fire source localization is in the two dimensional plane. As a consequence, problem of the three-dimensional fire sourceis dimensional plane. As a consequence, the problem the of the three-dimensional fire source localization essentially converted into a two-dimensional orientation determination of the center point of the flow localization is essentially into a two-dimensional orientationofdetermination of the center essentially converted into aconverted two-dimensional orientation determination the center point of the flow ofpoint the hot gases. Obviously a more complex model would be needed if, e.g., the fire would be located of the flow of the hot gases. Obviously a more complex model would be needed if, e.g., the fire of the hot gases. Obviously a more complex model would be needed if, e.g., the fire would be located near a wall or for non-symmetric configurations. However, as is demonstrated here, the proposed would be located a wall or forconfigurations. non-symmetric However, configurations. as is demonstrated here, near a wall or for near non-symmetric as is However, demonstrated here, the proposed method enables a fast and efficient fire source location in many situations. the proposed method enables a fast and efficient fire source location in many situations. method enables a fast and efficient fire source location in many situations. Ceiling Ceiling
Fire source Fire source
Figure 1. 1. A fire in aa closed closed room. Figure A fire fire in in Figure 1. A a closed room. room.
The two-dimensional fire source localization and geometrical arrangement of the optical fiber are The geometrical arrangement arrangementof ofthe theoptical opticalfiber fiberare are Thetwo-dimensional two-dimensionalfire firesource source localization localization and geometrical shown in Figure 2, which is the top view of Figure 1, for two different configurations. The closed rings shown for two two different differentconfigurations. configurations.The Theclosed closedrings rings shownininFigure Figure2,2,which whichisisthe the top top view view of Figure 1, for represent wavefronts of the hot gases, and each ring means a wavefront with identical temperature. represent means aa wavefront wavefrontwith withidentical identicaltemperature. temperature. representwavefronts wavefrontsofofthe thehot hot gases, gases, and and each ring means Point A is the fire source location. The wavefronts M and N represent two different radii of the circular Point represent two twodifferent differentradii radiiofofthe thecircular circular PointAAisisthe thefire firesource sourcelocation. location. The The wavefronts wavefronts M and N represent wavefront. Two sections of the optical fibers are placed parallel within the temperature field, crossing wavefront. Two sections of the optical fibers are placed parallel within the temperature field, crossing wavefront. Two sections of the optical parallel within the temperature field, crossing with different radii of wavefronts. The points B and D are the points of tangency between the fibers and with The points B and DD areare thethe points of of tangency between thethe fibers and withdifferent differentradii radiiofofwavefronts. wavefronts. The points B and points tangency between fibers the corresponding wavefronts. The assumption is made that the fire source is small enough so that it the corresponding wavefronts. The assumption is made that the fire is small enough so that and the corresponding wavefronts. The assumption is made that thesource fire source is small enough soit can be seen as a point fire source. Then the points with maximum temperature measured along the can asseen a point source. Then Then the points withwith maximum temperature measured along the thatbeit seen can be as a fire point fire source. the points maximum temperature measured along fibers are points B and D, respectively, and the smaller the radius, the higher the temperature. The fibers are points B and and and the smaller the the radius, the the higher the the temperature. The the fibers are points B D, andrespectively, D, respectively, the smaller radius, higher temperature. position of the high temperature point can be determined by using OTDR, which is the abscissa of the position of theofhigh temperature pointpoint can be by using OTDR, which is the abscissa of of the The position the high temperature candetermined be determined by using OTDR, which is the abscissa fire source. fire thesource. fire source. a a
y y
D D
N N
x x
M M
Fiber Fiber
Ⅱ
d d
Source A Source A
s= s=dd-r -r
r r B B
b b
Ⅱ
l l
y y C C
Fiber Ⅰ Fiber Ⅰ
N N
x x
M M
Source A Source A
r r
s= s=rr+d +d
B dB
l l
d
D D
C C
Fiber Ⅰ Fiber Ⅰ Fiber Fiber
Ⅱ Ⅱ
Figure 2. Fire source and the geometrical arrangement of the sensing fibers: (a) Fibers on opposite sides Figure 2.2.Fire source and thethe geometrical arrangement of the sensing fibers: (a) Fibers on opposite sides Fire and ofFigure fire source; (b)source Both fibers ongeometrical same side ofarrangement fire source. of the sensing fibers: (a) Fibers on opposite ofsides fire source; (b) Both fibers on same side of fire source. of fire source; (b) Both fibers on same side of fire source.
To determine the fire source location the ordinate of the fire source is required, which is the To determine the fire source location the ordinate of the fire source is required, which is the distance r between fiber I and the fire source. The measured temperature at point B is higher than the distance r between fiber I and the fire source. The measured temperature at point B is higher than the one at point D due to the smaller radius r of wavefront M compared to S of wavefront N. Obviously, one at point D due to the smaller radius r of wavefront M compared to S of wavefront N. Obviously, there is one point C at fiber I at which the measured temperature is the same as at point D at fiber II, there is one point C at fiber I at which the measured temperature is the same as at point D at fiber II, because points C and D belong to the same wavefront N. The distance between the two parallel fibers is because points C and D belong to the same wavefront N. The distance between the two parallel fibers is
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To determine the fire source location the ordinate of the fire source is required, which is the distance r between fiber I and the fire source. The measured temperature at point B is higher than the one at point D due to the smaller radius r of wavefront M compared to S of wavefront N. Obviously, there is one point C at fiber I at which the measured temperature is the same as at point D at fiber II, because points C and D belong to the same wavefront N. The distance between the two parallel fibers is d and l Sensors 829 is the2016, fiber16,length between point B and point C. Noticing the right triangle ABC the relationship can4 of be12 expressed as: d and l is the fiber length between point B and 2point2 C. Noticing the right triangle ABC the relationship r ` l “ s2 (3) can be expressed as: When the fire source is located between the two fibers (see Figure 2a), then: (3) r 2 l 2 s2 d´ r “fibers s When the fire source is located between the two (see Figure 2a), then:
d of rthe stwo fibers (see Figure 2b) we have: When the fire source is located on one side When the fire source is located on one side of the two fibers (see Figure 2b) we have: r`d “ s rd s Thus the distance radius r can be expressed as: Thus the distance radius r can be expressed as: ˇ 22 ˇ ˇl ´ dd2 2ˇˇ ˇ rr “ ˇ ˇ 2d 2d
(4)
(4) (5) (5)
(6) (6)
Withthe theabove aboveabscissa abscissaand andthe theordinate ordinateofofthe thefire firesource, source,the thelocalization localizationofofthe thefire firesource sourceis With is defined. defined. Experimental Setup 3.3.Experimental Setup Theschematic schematicdrawing drawing DTS system is presented in Figure 3. short The short of from light a The of of thethe DTS system is presented in Figure 3. The pulsepulse of light from afiber pulsed laser operating 1550a nm with a peak maximum W, width 10 ns pulse pulsed laserfiber operating at 1550 nmatwith maximum powerpeak of 30power W, 10 of ns 30 pulse and 10 width and 10 kHz repetition rate is injected into the optical sensing fiber through a 1 ˆ 3 wavelength kHz repetition rate is injected into the optical sensing fiber through a 1 × 3 wavelength division division multiplexer (WDM).scattering Raman scattering in thesensing optical sensing fiber, the backscattered multiplexer (WDM). Raman occurs inoccurs the optical fiber, and theand backscattered Raman RamanasStokes the Raman anti-Stokes passing through WDM injectedinto into a Stokes well as as well the as Raman anti-Stokes light light passing through the the WDM areareinjected a high-performance InGaAs avalanche photodiode (APD). Using the APD, the Raman Stokes high-performance InGaAs avalanche photodiode (APD). Using the APD, the Raman Stokesand and anti-Stokeslight lightare aretransformed transformed into into electrical electrical signals. signals. Then anti-Stokes Then aa high high speed speed100 100MHz MHzdata dataacquisition acquisition cardconverts convertsthe theanalog analogsignals signals to to digital digital signals. card signals. Finally, Finally, the thecomputer computerprocesses processesthe thedigital digitalsignals signals and calculates the temperature data according to the theoretical analysis presented in Section 2.1. and calculates the temperature data according to the theoretical analysis presented in Section 2.1. Laser
1×3 WDM Sensing Fiber
APD
SYNC
APD
DAQ
Figure setup (WDM: (WDM: wavelength wavelengthdivision divisionmultiplexer; multiplexer;APD: APD:InGaAs InGaAs Figure3.3.Schematic Schematicof ofthe the DTS DTS system system setup avalanche photodiode; DAQ: data acquisition). avalanche photodiode; DAQ: data acquisition).
To verify the theoretical analysis fire source localization experiments were carried out in a To verify the theoretical analysis fire source localization experiments were carried out in a laboratory in a simulated environment. The experimental setup is shown in Figure 4. A tray of alcohol laboratory in a simulated environment. The experimental setup is shown in Figure 4. A tray of (11.0 cm × 8.0 cm × 2.1 cm) placed on a table (2.4 m × 1.2 m × 0.8 m) is used as the fire source. In the alcohol (11.0 cm ˆ 8.0 cm ˆ 2.1 cm) placed on a table (2.4 m ˆ 1.2 m ˆ 0.8 m) is used as the fire closed laboratory room, the hot gases rise to the ceiling and propagate under the ceiling. Then the hot source. In the closed laboratory room, the hot gases rise to the ceiling and propagate under the gases flow down at the walls and flow back to the fire source origin near floor level. Thus there is a hot flue gas layer with a thickness of 100–300 mm beneath the ceiling if the fire source is located far away from the walls. In order to simplify the experiments, a heat insulation roof (2.4 m × 1.2 m × 0.05 m) supported by four pillars is placed 1 m above the table. This roof is placed parallel to the table and acts as the “ceiling” as discussed above in Figure 1. The table, the four pillars and the ceiling form an open
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ceiling. Then the hot gases flow down at the walls and flow back to the fire source origin near floor level. Thus there is a hot flue gas layer with a thickness of 100–300 mm beneath the ceiling if the fire source is located far away from the walls. In order to simplify the experiments, a heat insulation roof (2.4 m ˆ 1.2 m ˆ 0.05 m) supported by four pillars is placed 1 m above the table. This roof is placed parallel to the table and acts as the “ceiling” as discussed above in Figure 1. The table, the four pillars and the ceiling form an open cabinet in the laboratory. If a fire occurs in the cabinet, the hot gases produced by the fire rise up to the heat insulation roof and propagate under the roof, forming a layer of the2016, hot16, flue of12 Sensors 829gases beneath the roof. Because the open cabinet is placed far away from the walls 5 of the closed laboratory room, the hot gases propagating to the edge of the roof will continue to flow horizontally of flowing thethe room walls. There is layer no influence on roof. the when arrivinginstead at the room walls.down Therewhen is no arriving influenceaton inner hot flue gas under the inner hot flue layer is under the roof. As a result, the cabinet is of employed to simulate true situation As a result, thegas cabinet employed to simulate a true situation a fire with sensors aplaced far away of a the fire edge with sensors placed far awaymultimode from the edge the roof. A standard with from of the roof. A standard fiberofwith an attenuation ofmultimode
